1. Field of the Invention.
This invention relates to apparatus for measuring the variation or non-linearity in the velocity profile of a system under test.
2. Description of the Prior Art.
Various prior art systems are known which provide an encoded electrical signal in response to mechanical motion. These systems, of which U.S. Pat. Nos. 4,122,332, 3,731,301, 3,733,472, and 3,885,420 are representative, are generally complex systems requiring many electronic components. Wilkenson, in U.S. Pat. No. 4,122,332, provides a shaft encoder which produces an encoded electrical signal in accordance with rotation of a shaft. The electrical signal is further processed through a plurality of code converters, before being coupled to a comparator which receives a converted reference signal at its other input. The two inputs of the comparator are thereby analog signals, and the output is simply indicative of the condition wherein one signal is greater than or equal to the other signal. The comparator output is further processed and utilized to activate a warning unit.
The Davis patent, U.S. Pat. No. 3,731,301, shows a system for providing a control signal indicative of the deviation of the state of movement of a jet engine shaft. This system stores and subsequently provides the difference between two successive samples, if the difference between two successive samples is more than a predetermined amount. Thus, this system requires a plurality of memory devices to cyclically sample and store successive encoded signals. Therefore, this system simply achieves a continuous comparison between encoded signals, to provide a control signal, but does not contemplate either comparison with a predetermined reference signal or the precise display of the exact difference.
A similar system which requires a plurality of memory circuits, and extensive digital circuitry, is shown by U.S. Pat. No. 3,733,472 to Taisne. This system detects counting errors in an encoded output signal, by performing a plurality of discrimination and differencing operations. The difference between a sensed value and a registered value is memorized at the beginning of measurement, and subsequent sensed values are continuously compared with the memorized difference. The system detects noncoincidence between the instantaneous difference and the memorized difference for the purpose of actuating an alarm.
The Wolfinger patent, U.S. Pat. No. 3,885,420 shows a system for measuring small variations in the speed of rotating shafts. This system compares a zero-crossing detected shaft signal with an oscillator reference signal via a phase detector. The phase detected output signal is further subjected to low-pass filtering and notch filtering to produce a signal having an amplitude and frequency corresponding to torsional vibration of the shaft.
It can be seen that the prior art systems are relatively complex, and yet do not provide a specific digital output signal representative of speed variation, or non-linearity of velocity. Further, no prior art system has accomplished the measurement of velocity differences in as simple and direct a manner as is achieved by the present invention.
Accordingly, it is the main object of the present invention to provide a digital electronic circuit of simple construction for measuring the exact deviation of a detected velocity from a reference velocity.
Another object of the invention is to provide an electronic frequency deviation monitor which gives an instantaneous visual indication of the exact deviation from a nominal frequency.
Other objects and advantages of the invention will be apparent from the following description and appended claims.